An absorption amount of a poorly water soluble drug is small in many cases since the drug is not sufficiently dissolved in a digestive tract when orally administered. In such drug, an absorption ratio of the drug is decreased with the increase of administration amount, and the absorption ratio varies more easily due to digestion activity at fed state (mechanical stimulation due to constriction movement of digestive tract, increase in secretion amount of digestive fluid, prolongation of digestive tract retention time etc.) as compared with at fast state, the expected therapeutic effect is not obtained, or accidental adverse event is caused in some cases. For this reason, particularly, it is important for developing an oral preparation to enhance solubility of the drug from the solid preparation.
As one of means to improve solubility of the poorly water soluble drug in an aqueous liquid from the solid preparation, a solid dispersion in which a drug molecule is uniformly dispersed in inactive ingredients in the solid state is used.
A preparation containing nifedipine which is a poorly water soluble drug as such the solid dispersion, hydroxypropylmethylcellulose and polyvinylpyrrolidone as a water-soluble polymer, and 60% by weight of nicotinic acid amide as an additive is disclosed (Non-Patent Literature 1).
As a method of producing the solid dispersion, a process comprising heating or mechanochemically treating a preparation containing nicardipine hydrochloride, hydroxypropylmethylcellulose and 15% by weight of urea is disclosed (Patent Literature 1). Like this, nicotinic acid amide and urea are known as an additive for forming the solid dispersion.
However, in Non-Patent Literature 1, even when the water-soluble polymer is changed, the poorly water soluble drug has been dissolved out only by about a half amount at maximum. The process of Patent Literature 1 is a process of giving an excessive stress for the drug such as heating at a high temperature and mechanochemical treatment of the preparation. Therefore, any technique is not sufficient for the preparation of the solid dispersion. In addition, when a content of the drug in the preparation is high, solubilizing effect of the solid dispersion is reduced.    Non-Patent Literature 1: Chem. Pharm. Bull., vol. 46, 482-487, 1998    Patent Literature 1: International Publication Pamphlet    WO 97/06781